In refrigerating systems, a positive displacement compressor such as a piston type compressor is typically used to compress refrigerant gas from a lower suction pressure value to a much higher discharge pressure. Such compressors include a crankcase as a storage location for the lubricating oil. A suction conduit connects an evaporator or cooling coil to the compressor inlet.
Positive displacement type compressors can easily be damaged should liquid refrigerant rather than vapor refrigerant be drawn either into its crankcase or its compression chamber. The term floodback is employed to denote and describe the condition when liquid refrigerant flows through the suction conduit and enters the crankcase or cylinders of a compressor. xe2x80x9cSluggingxe2x80x9d is the term used to describe the condition when so much incompressible liquid refrigerant enters the compressor cylinders or other types of compression chambers that audible noise of chattering or hammering occurs. In larger compressors, instantaneous compressor damage or destruction can occur under slugging conditions.
Even if insufficient liquid refrigerant enters the running compressor to cause slugging, compressor damage can occur if the lubricating oil in the crankcase is diluted by liquid refrigerant entering the crankcase or oil sump, thereby reducing its viscosity and lubricity and generating excessive wear of the lubricated parts such as main and connecting rod bearings and cylinder walls and piston rings. While this kind of damage may not be immediately observable, it always leads to shortened compressor life and progressively noisier operation. The debris of excessive wear can circulate around the system with the refrigerant plugging driers and filters and reducing system performance through increased pressure drop through these partially plugged components.
Even with the compressor xe2x80x98offxe2x80x99, migration of gaseous refrigerant to an unusually cold crankcase there, mixing with the oil, can cause both immediate and long term compressor destruction when the compressor starts up or runs under this condition. The dual destructive sequence begins when the compressor starts, sharply reducing the pressure within the crankcase and thereby causing the excessively refrigerant laden lubricant to foam and to be drawn into the compressor cylinders causing immediate slugging. Even if slugging does not occur, the reduced lubricity of the diluted oil promotes excessive wear and early compressor failure.
During compressor operation, the floodback condition of liquid refrigerant flowing to the compressor can be caused by incorrect expansion valve setting or expansion valve or restrictor malfunction or by loss or excessive reduction of evaporator load. Expansion valves are devices that control or restrict refrigerant liquid flow into the evaporator to just the amount or rate of flow that the evaporator can evaporate. Reductions in evaporator load can be caused by plugged filters in the evaporator airstream or frost clogging the evaporator face or failure of the evaporator fan/s to operate.
In the past various controls and piping artifacts have been used to try to ensure that the refrigerant being compressed is always in gaseous form, that is, that no floodback to the compressor occurs. One such refrigerant flow control is a thermostatic expansion valve that senses the superheat condition at the evaporator outlet and adjusts refrigerant flow in response. Piping artifacts employed to help ensure that only gas flows to the compressor include suction accumulators and suction traps; these are vessels that catch and retain liquid refrigerant while allowing refrigerant vapor to continue to flow to the compressor.
Service personnel on the site can detect floodback to the compressor while it occurs by employing rough indicia such as frost formation on the suction line, though this indication must be skillfully used since suction frost can be formed by dry gas (no liquid) that is simply colder than 32 F.
Therefore mechanics developed their own tools for deciding whether there was liquid refrigerant within a suction line. They would grip the bare line and judge how fast their hand chilled. We now know the high heat transfer coefficient between liquid refrigerant and the pipe makes the hand holding the pipe feel cool, and appear to cool, more rapidly than a similar pipe carrying only cold vapor with no liquid refrigerant in it.
Other mechanics would wet their finger tips and touch the pipe, thereby deciding on the presence or absence of liquid in the pipe by whether or not the wet finger froze to the pipe. Other mechanics would listen to the sound of the operating compressor to help them decide whether the compressor was attempting to compress a mixture of refrigerant vapor and liquid or vapor only.
However, even normal systems have a substantial potential for xe2x80x98noisexe2x80x99, that is, xe2x80x98huntingxe2x80x99 or cyclic or random variations in suction line temperature and pressure. These random changes could be caused by normal fluctuations in thermal expansion valves or by sharp load changes generated by momentary fans-off condition or other normal but transient events. Therefore early efforts to utilize suction line temperatures as predictors of floodback were ineffective and it became xe2x80x98common knowledgexe2x80x99 that protective measures based on suction line temperatures were unreliable.
None of these sensing methods was amenable to automatic mechanical or electrical sensing whereby a floodback condition hazardous to the life of the compressor could be observed and the compressor turned off or an alarm signaled.
Superheat measurement at the suction inlet of the compressor can indicate the presence of liquid refrigerant. However, to be effective superheat measurements must be made skillfully and with the proper equipment correctly applied.
Superheat is accurately measured only by measuring the pressure of the suction gas, determining the saturation temperature from a pressure/saturation temperature conversion chart, generally by interpolation, or by evaluation of an equation that simulates the pressure-temperature curve for that refrigerant. Then the calculated saturation temperature is subtracted from the observed suction line temperature, the difference being the superheat.
Among service personnel it is not well known that the commonly used Bourdon pressure gages that compare line pressure with atmospheric pressure are subject to variations from altitude and weather that can affect the expected saturation temperature, especially in freezer systems. Sufficiently accurate pressures can be observed employing xe2x80x98absolutexe2x80x99 pressure detectors. Such pressure detectors employ the pressure in a highly evacuated chamber as the reference against which the suction pressure is measured, instead of atmospheric pressure. While the difference between zero superheat, indicating a floodback condition, and a small positive superheat, a safe operating condition, is difficult to accurately determine accurately because of the need to interpolate temperature values from the ubiquitous service xe2x80x98pressure-temperaturexe2x80x99 chart, accurate corresponding saturation temperatures can readily and quickly be generated from digitized stored data or from stored equations.
Barbier Patent Number 5,627,770
This is simply a superheat indicating device. It does not disclose any provision for detecting floodback as such. Further, the specification points out at col.2 139-44 that saturated temperatures are almost always employed rather than actual temperatures. It does not disclose or suggest any rate function.
Aloise Patent Number 5,666,815
Aloise teaches the apparatus and method for storing the vapor pressure/temperature models for a number of refrigerants in the integral microprocessor, switch selecting the appropriate refrigerant, observing the desired system temperature and pressure, calculating the saturated temperature for the refrigerant selected, and subtracting the calculated temperature from the observed temperature. Aloise does not discuss any rate function.
Lockhart Patent Number 5,311,745
Lockhart discloses a process for digitally observing and displaying refrigerant pressure and corresponding temperature. Further, he discloses a process for storing sequential pressure data and displaying the direction of pressure change. (Col 1, lines 55-68) He does not teach measurement of temperature for comparison with the observed pressure or calculated corresponding temperature.
Kauffman Patent Number 5,209,076
Kauffman takes a xe2x80x98shot-gunxe2x80x99 approach to compressor protection by observing a multitude of operational parameters including suction superheat, establishing a tolerable range for the parameters and shutting down the compressor in the event one or more of the observed parameters fall outside the pre-established limits. While the disclosure suggests the storage of a series of data points and presentation of xe2x80x98trendsxe2x80x99, it does not suggest any particular action be taken with respect to the observed trends, nor, in particular does it suggest any immediate action be taken with respect to any particular rate function.
The specification discloses novel equipment and processes:
for observing the value and the rate of change of suction superheat near the inlet of a refrigeration compressor;
for observing the value and rate of change of suction temperature at or near the inlet of a refrigeration compressor;
for establishing and storing suction temperature limits;
for establishing and storing suction temperature rate of change limits;
for establishing and storing suction superheat limits
for establishing and storing suction superheat rate of change limits
for comparing observed suction temperature and its rate of change with the established and stored rates;
for comparing observed suction superheat and its rate of change with the established and stored rates;
and for taking action protective of the compressor when either or both values or rates are repeatedly equal to or exceed the stored rates over a time period.
It is a primary object of the present invention to provide means for protecting a refrigeration compressor against liquid refrigerant floodback.
It is a further object to provide such protection by measuring the temperature of the suction stream approaching the compressor at predetermined intervals.
It is a further object to provide such protection by measuring the pressure of the suction stream approaching the compressor at times substantially coincident with the times of the temperature measurement.
It is a further object to provide such protection by measuring the pressure of the suction stream at times not substantially coincident with the times of the temperature measurement.
It is a further object to measure such suction pressure employing means of measuring absolute pressure including piezo-resistive pressure measuring means.
It is a further object to provide stored data for calculating saturated refrigerant temperature of the employed refrigerant from the observed suction pressure.
It is a further object to provide such calculating means where the means include a stored model of the refrigerant characteristics.
It is a further object to provide the model on a solid state plug-in memory storage device.
It is a further object to calculate the superheat of the refrigerant suction stream by subtracting the calculated saturated refrigerant temperature corresponding to the coincidently observed refrigerant pressure from the observed suction temperature.
It is a further object to calculate the superheat of the refrigerant suction stream by subtracting the calculated saturated refrigerant temperature, corresponding to refrigerant pressure, from the suction temperature observed at a time later than the time of the pressure measurement from which the saturated refrigerant temperature is calculated.
It is a further object to store a sequence of the periodically observed suction temperatures.
It is a further object to calculate a temperature rate of change of the suction stream from some of the periodically observed temperature values.
In an air conditioning application, it is a further object to activate a control means when the measured temperature of the suction line falls below a set temperature.
In an air conditioning application is it a further object to disable the activation of the control means by temperature when the ambient temperature is lower than a set temperature and in the alternative to enable activation of the control means when the suction temperature rate-of-change exceeds a set value.
It is a further object to store a sequence of the periodically calculated corresponding suction superheats.
It is a further object to calculate a superheat rate of change from some of the periodically calculated superheat values.
It is a further object to provide means for inputting and for storing a predetermined limiting value of superheat.
It is a further object to provide means for inputting and for storing a predetermined limiting value of superheat rate of change.
It is a further object to provide means for inputting and for storing a predetermined limiting value of temperature.
It is a further object to provide means for inputting and for storing a predetermined limiting value of temperature rate of change.
It is a further object to provide means for determining from prior observations such limiting values and for storing them.
It is a further object to provide means for comparing observed temperature values with the stored limiting temperature values.
It is a further object to provide means for comparing calculated superheats with the stored limiting superheat values.
It is a further object to provide means for comparing calculated superheat rates of change with the stored limiting superheat rate of change values.
It is a further object to provide control means for operating alarms and/or compressor protective devices in response to observed temperatures or temperature rates of change or in response to calculated superheats or superheat rates of change that differ from or that exceed, positively or negatively, established or stored rates of change.
And further including the following objects:
To protect an air conditioning or refrigeration compressor from the harmful effects of liquid floodback of an employed refrigerant by providing a stored model of the employed refrigerant saturated characteristics;
Making frequent time based observations of suction temperature and suction pressure near the compressor inlet;
Employing the observed pressure and stored model of the refrigerant saturated characteristics to determine suction superheat from each observation; Determining the time rate of change of both suction temperature and suction superheat;
Establishing and storing rating-of-change limits;
Comparing the observed rates of change with the established rates and taking compressor protective measures when the observed rates exceed the established stored rates.
Storing, but not acting on a first observation of a hazard condition, setting a timer to zero, establishing a time limit, acting on a second observation of a hazard condition within the time limit, erasing the stored first observation of a hazard condition on expiration of the time limit if a second hazard condition has not occurred.